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| United States Patent |
5,349,833
|
|
Pardee
, et al.
|
September 27, 1994
|
Cryotrap for air pollution analyzer
Abstract
A cryotrap for an air pollution analyzer having a core with an axial
opening for receiving a cooling gas expander, a length of tubing carried
on the core defining a gas flow path around the gas expander, and an
electric heater on the core, with the tubing disposed between the heater
and expander. The core has a helical groove around the exterior thereof,
with the length of tubing positioned in the groove, and the heater
positioned around the core over the tubing. A Stirling linear drive cooler
charged with helium and having a gas compressor connected to one end of
the gas expander with the other end of the expander positioned in the
axial opening of the core.
| Inventors:
|
Pardee; Michael A. (Saugus, CA);
Yoong; Matthias J. C. (Ventura, CA)
|
| Assignee:
|
Xontech, Inc. (Van Nuys, CA)
|
| Appl. No.:
|
020981 |
| Filed:
|
February 22, 1993 |
| Current U.S. Class: |
62/55.5; 73/863.11 |
| Intern'l Class: |
B01D 008/00 |
| Field of Search: |
62/55.5
73/863.11
|
References Cited
U.S. Patent Documents
| 2880615 | Apr., 1959 | Hardy et al.
| |
| 3485054 | Dec., 1969 | Hogan | 62/55.
|
| 3611812 | Oct., 1971 | Cleveland.
| |
| 3673871 | Jul., 1972 | Randle et al.
| |
| 3712074 | Jan., 1973 | Boissin | 62/55.
|
| 3881359 | May., 1975 | Culbertson.
| |
| 4148196 | Apr., 1979 | French et al. | 62/55.
|
| 4154088 | May., 1979 | Werner.
| |
| 4283948 | Aug., 1981 | Longsworth | 62/55.
|
| 4425811 | Jan., 1984 | Chatzipetros et al. | 62/55.
|
| 4479927 | Oct., 1984 | Gelernt | 62/55.
|
| 4506513 | Mar., 1985 | Max.
| |
| 4607491 | Aug., 1986 | Ishumuru et al. | 62/55.
|
| 4677863 | Jul., 1987 | Gay et al. | 62/55.
|
| 4964278 | Oct., 1990 | Wen et al.
| |
| 4977749 | Dec., 1990 | Sercel.
| |
| 5073896 | Dec., 1991 | Reid et al. | 62/55.
|
Primary Examiner: Bennett; Henry A.
Assistant Examiner: Kilner; Christopher
Attorney, Agent or Firm: Harris, Wallen MacDermott & Tinsley
Claims
We claim:
1. In a cryotrap for an air pollution analyzer, the combination of:
a core having first means for receiving a cooling gas expander;
a length of tubing carried on said core defining a gas flow path around
said gas expander; and
an electric heater on said core, with said tubing disposed between said
heater and expander;
said core having a helical groove around the exterior thereof, with said
length of tubing positioned in said groove, and with said heater
positioned around said core over said tubing.
2. A cryotrap as defined in claim 1 wherein said core is a cylindrical
copper rod with a helical groove around the exterior with said tubing
wound therein, with an axial opening for said gas expander, and including
a copper foil wrapped around said core over said heater.
3. A cryotrap as defined in claim 2 including a second opening in said core
between said axial opening and said helical groove for receiving a
temperature sensor.
4. A cryotrap as defined in claim 1 including a Stirling linear drive
cooler charged with helium and having a gas compressor connected to one
end of said gas expander with the other end of said expander positioned in
said core.
5. In a cryotrap for an air pollution analyzer, the combination of:
a core having first means for receiving a cooling gas expander;
a length of tubing carried on said core defining a gas flow path around
said gas expander;
an electric heater on said core, with said tubing disposed between said
heater and expander; and
a support for said core having a first central plate and a plurality of
second insulator plates, with at least one second plate on each side of
said first plate, and with said core positioned in said first plate.
6. A cryotrap as defined in claim 5 wherein each of said first and second
plates is of a low density, closed cell, rigid foam thermal insulation
with a plurality of insulator openings therethrough, and metalized plastic
film on each side of the plate.
7. A cryotrap as defined in claim 6 including crumpled metalized plastic
film filling said insulator openings of said second plates.
8. A cryotrap as defined in claim 7 including a plurality of third
insulator plates with at least one third plate on each side of the first
and second plate combination, and a fourth mounting plate on a side of the
first, second and third plate combination; and
means for joining said plates in a sandwich construction;
with each of said third plates of a low density, closed cell, rigid foam
thermal insulation and an metalized plastic film on each side of the
plate; and
with aligned expander openings through the second, third and fourth plates
on one side of said first plate aligned with said core in said first plate
for receiving said gas expander.
9. A cryotrap as defined in claim 8 wherein said insulator openings in
adjacent plates are out of alignment with each other.
10. A cryotrap as defined in claim 9 wherein said plates are in the order
of 1/2 inch thick and square and about 4 inches long on a side.
11. In a cryotrap for an air pollution analyzer, the combination of:
a core having a first opening for receiving a cooling gas expander and a
helical groove around the exterior thereof;
a length of tubing carried on said core in said helical groove and defining
a gas flow path around said gas expander;
an electric heater on said core, positioned around said core over said
tubing;
a copper foil wrapped around said core over said heater; and
a support for said core having a first central plate and a plurality of
second insulator plates, with at least one second plate on each side of
said first plate, and with said core positioned in said first plate,
with each of said first and second plates of a low density, closed cell,
rigid foam thermal insulation with a plurality of openings therethrough,
and a metalized plastic film on each side of the plate, and with crumpled
metalized plastic film filling said openings of said second plates.
12. A cryotrap as defined in claim 11 including a Stirling linear drive
cooler charged with helium and having a gas compressor connected to one
end of said gas expander with the other end of said expander positioned in
said axial opening of said core.
13. A cryotrap as defined in claim 12 including a plurality of third
insulator plates with at least one third plate on each side of the first
and second plate combination, and a fourth mounting plate on a side of the
first, second and third plate combination; and
means for joining said plates in a sandwich construction;
with each of said third plates of a low density, closed cell, rigid foam
thermal insulation and a metalized plastic film on each side of the plate;
and
with aligned openings through the second, third and fourth plates on one
side of said first plate aligned with said axial opening of said first
plate for receiving said gas expander.
Description
BACKGROUND OF THE INVENTION
This invention relates to air pollution analyzers and the like which
measure constituents in a sample gas stream. More particularly, the
invention relates to a cryotrap used for extracting constituents from the
sample gas.
In a typical air pollution application, the air sample is passed through a
cryotrap which is maintained at the temperature of liquid nitrogen or
other liquid cryogens. Constituents in the gas sample are frozen by the
low temperature and solidify on the cold surfaces of the trap. After the
desired amount of sample gas has passed through the trap, the gas flow
path is changed so as to pass a carrier gas through the trap in place of
the sample gas. At this time, the trap is heated. The frozen constituents
vaporize and are carried away with the carrier gas to an analyzer or other
instrument. The now clean trap is ready for another sampling and flushing
cycle.
In the cryotraps commonly used, liquid nitrogen is the gas chosen for the
freezing step. However, the liquid nitrogen is lost to the atmosphere and
a new quantity must be utilized for each operation. For instruments which
operate continuously, the cost of liquid nitrogen as the freezing gas is
very high.
It is an object of the present invention to provide a new and improved
cryotrap which uses a closed cycle cooler for the freezing with no coolant
being discharged after each sampling cycle.
A further object of the invention is to provide such a cryotrap with a new
and improved construction for obtaining increased efficiency in cooling
and heating while enabling a smaller construction and shorter operating
cycles.
Earlier mechanical coolers are bulky, heavy, noisy, and have high
mechanical vibration. They have relatively short life cycles and do not
cool to liquid nitrogen temperature. It is an object of the present
invention to use a Stirling closed cycle linear cooler which is relatively
lightweight and compact. Features of the Stirling cooler include low
conducted and radiated emissions, low mechanical vibration, acoustically
quiet operation, and improved lifetime and reliability.
Other objects, advantages, features and results will more fully appear in
the course of the following description.
SUMMARY OF THE INVENTION
The cryotrap of the invention includes a core having first means for
receiving a cooling gas expander, a length of tubing carried on the core
defining a gas flow path around the gas expander, and an electric heater
on the core, with the tubing disposed between the heater and expander. The
core preferably has a helical groove around the exterior thereof, with the
tubing positioned in the groove, and with the heater positioned around the
core over the tubing. The core may be a cylindrical copper rod with a
helical groove on the exterior with the tubing wound therein, and an axial
opening for the gas expander, and including a copper foil wrapped around
the core over the heater.
The source of cooling may comprise a Stirling linear drive cooler charged
with helium and having a gas compressor connected to one end of the gas
expander with the other end of the expander positioned in the core.
Insulation for the cryotrap preferably includes a support for the core
having a first central plate and a plurality of second insulator plates,
with at least one second plate on each side of the first plate, and with
the core positioned in the first plate. Each of the first and second
plates is of a low density, closed cell, rigid foam thermal insulation
with a plurality of openings therethrough, and metalized plastic film on
each side of the plate, and includes crumpled metalized plastic film
filling the openings of the second plate. A plurality of third insulator
plates may be used with at least one third plate on each side of the first
and second plate combination, and a fourth mounting plate on a side of the
first, second and third plate combination with the plates joined in a
sandwich construction.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded view of a cryotrap incorporating the presently
preferred embodiment of the invention;
FIG. 2 is an enlarged and exploded view of the core and coil of the
cryotrap of FIG. 1;
FIG. 3 is a side view of the core plate of FIG. 1;
FIG. 4 is an enlarged sectional view taken along the line 4--4 of FIG. 3;
FIG. 5A is a diagram illustrating the operation of the cryotrap, with the
control valve in the sampling position; and
FIG. 5B is a diagram similar to that of FIG. 5A with the control valve in
the flushing position; and
FIG. 6 is an electrical block diagram for the cryotrap.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the cryotrap shown in FIG. 1, a core plate 11, apertured insulator
plates 12, solid insulator plates 13, and a mounting plate 14 are held
together in a sandwich construction by rods 15 and screws 16 positioned at
the corners of the plates. A core 19 is positioned in the core plate 11
and a gas expander 20 is attached to the mounting plate 14 by screws 21,
with the tube 22 of the gas expander extending through openings 23 in the
plates and into an axial opening 24 in the core 19. A thermally conductive
compound such as silver-loaded silicone grease may be applied to the end
of the expander tube 22 to improve the thermal connection to the core 19.
The gas expander is part of a refrigeration system which provides for
cooling of the core 19. In the preferred embodiment illustrated, a
Stirling linear cooler is utilized, having a compressor 27 connected to
the gas expander by a line 28. The Stirling linear cooler may be of
conventional design, providing a closed cycle with helium being compressed
at the compressor and with pressure pulses transferred through the helium
to the expander. Cooling is obtained by cyclic out-of-phase motion of a
compression piston and a displacer-regenerator located in the expander
assembly.
The compressor is operated for a time prior to the introduction of the
sample to allow the core and tubing to reach the desired operating
temperature. The compressor continues to operate during the time that the
sample is passed through the trap tubing. The compressor is then turned
off during the heating mode.
The preferred construction for the core and the plates is shown in greater
detail in FIGS. 2-4. The core 19 preferably is a copper cylinder 30 with a
helical groove 31 on the exterior. A length of tubing 32, preferably of
stainless steel, is wound on the core in the helical groove. Temperature
sensors 33a, 33b may be positioned in openings 34 in the core, preferably
between the gas expander opening 24 and the helical groove B1. The
temperature sensors are connected to a control circuit as shown in FIG. 6
by wires 35.
A flexible heater 36 is placed on the core over the tubing and typically is
a Kapton film heater consisting of a resistive metallic foil heater
element insulated on both sides by a thin film of Kapton polyimide. A
layer of aluminum foil is bonded to the back of the heater to distribute
the heat evenly and a layer of pressure sensitive adhesive is applied to
the aluminum. The pressure sensitive adhesive serves to temporarily hold
the heater to the core. Copper foils with a pressure sensitive adhesive on
one side are then wrapped around the heater and core and soldered in
place. The copper foil serves to hold the heater in place.
During the cooling mode the compressor control electronics hold the core
temperature to a selectable preset temperature as sensed by the
temperature sensor 33a. During the heating mode the heater temperature
controller 52 holds the core temperature to a selectable preset
temperature as sensed by the temperature sensor 33b. The heater
temperature controller also provides a display of the core temperature.
Cooling and heating modes are controlled by the cryotrap sequence control
electronics 53, with power from adc power supply 54.
Desirably, the core plate 11 and insulator plates 12, 13 are formed of a
low density, closed cell, rigid foam for thermal insulation, typically a
polymethacrylimide foam. The individual plates preferably are about 1/2"
thick and about 4".times.4" square. A layer of moralized plastic film 40,
typically aluminized Mylar film, is positioned on each side of each plate,
typically about 0.0005" thick.
A plurality of openings 41 are provided in each of the core plate and
insulator plates, typically about 1/2" in diameter. Prior to applying the
surface layers 40, the openings preferably are filled with crumpled
metalized plastic film 42, typically aluminized Mylar film. Also,
preferably the openings 41 in adjacent plates are misaligned, as seen in
FIG. 1.
In operation, the core is cooled by the gas expander rod 22 in the opening
24 of the core. Substantial insulation is provided around the core and the
gas expander tube so there is minimal heating from the surrounding
atmosphere. At the same time, heat is periodically applied to the tubing
for thawing the frozen gas constituents. Hence, good heat transmission
between the heater and the tubing is desired. It has been found that the
insulation construction using the foam plates, with the crumpled metalized
film in the openings 41 achieves an excellent balance between heating and
cooling, permitting freezing of substantially all constituents in the gas
while at the same time requiring a minimum of cooling energy and
permitting a rapid cycle time.
It is desirable to cool the core quickly. Rapid cooling minimizes wear on
the Stirling cooler and enables the cryotrap to complete a cooling/heating
cycle quickly. To cool the core quickly it is necessary to minimize the
amount of material that must be cooled. This includes the insulating
material around the core, therefore insulating material is removed
resulting in a plurality of openings 41 in each of the core plates and
insulator plates. The openings are typically 1/2" in diameter leaving a
"web" of foam insulation to support the core and providing a longer path
through the foam from the core to the outside. To prevent air currents
from circulating in these openings, the openings are filled with crumpled
metalized plastic film 42, typically aluminized Mylar film. Metalized
plastic film is also bonded to each side of the insulating plate. The
metalized plastic film also serves to reflect external radiant energy away
from the core.
The cryotrap is utilized with a six port valve 44 which is movable between
a sampling position shown in FIG. 5A and a flushing position shown in FIG.
5B. A source of sample gas is connected to the valve through a line 45,
and a vent for the sample gas is connected to the valve through another
line 46. The carrier gas from an analyzer such as a gas chromatograph is
connected to the valve by an incoming line 47 and an outgoing line 48. The
tubing 32 is connected to the valve 44 by lines 49, 50.
During the sampling mode, the sample gas flows through the line 45, the
valve 44, through the tubing 32, and back through the valve to the vent
line 46, with the carrier gas flowing directly to and from the valve
through lines 47, 48.
With the valve turned to the flushing mode, the sample gas flows directly
into and out of the valve, with the carrier gas flowing through the line
47 to the valve, through the tubing 32 and back to the analyzer through
the valve and the line 48.
The operation of the valve may be automatic, operating on a predetermined
cycle or may be manually operated as desired.
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